HP Z620 - Geforce 970 black screen od death - power issues?

I have a GF 970 which demands a 8pin and a 6pin pcie as the Z820 only supports 2x6pin I used an adpater 6->8

I read somewhere that the 6pin HP Z620 will deliver more then 150W which is the rated power for a  8pin

now I have the issue that in some games my  screen is going black at start and I can still hear some sounds but basically I need to hard reset the PC then

In these games it happens all the time e.g. GTA V

other games are fine